The invention relates to a graphic tablet, including:
an array of substantially parallel electrodes, and PA1 a control circuit for presenting a series of successive patterns to the electrodes during a detection period, each pattern representing a signal value from among at least two standard values for each electrode, in order to form for each electrode, by way of said presentation, a series of successive code elements which is representative of the relevant electrode. PA1 a signal sensor which is displaceable relative to the electrodes in order to pick up signals to be generated by the electrodes in response to a given pattern, and PA1 a detection circuit which is coupled to the signal sensor in order to determine, on the basis of the signals picked up, an instantaneous position of the signal sensor relative to the electrodes during the detection period, the tablet including a measuring element for measuring an instantaneous noise level, the detection circuit being arranged to compare the instantaneous level of the signal picked up in the signal sensor with the measured instantaneous noise level, and the control circuit being arranged to execute the interpolation step on the basis of this comparison. The control circuit can then present patterns which contain increasingly smaller sub-series wherefrom the instantaneous position of the signal sensor can be determined with an increasing resolution until the instant at which the signal from the sub-series becomes insufficiently strong, in relation to the noise level, to enable reliable determination. Subsequently, an interpolation step takes place with the last pattern. As a result of the measurement of the instantaneous noise level at the tablet, the control circuit can continue the presentation of increasingly finer patterns as long as possible and proceed with the interpolation step only when necessary. As a result, the instantaneous position of the signal sensor can be determined in a minimum number of steps, adapted to the circumstances.
A tablet of this kind is known from European Patent Application EP 622 754 A1. The tablet is used inter alia as a graphic input apparatus via which given data is presented to a data processing system. To this end, the known tablet includes an array of parallel arranged electrodes and a signal sensor in the form of a pen which can be displaced relative to the electrodes. The relative position of the pen with respect to the electrodes constitutes one of the data presented to the data processing system by the tablet. The known tablet includes a control circuit which presents given patterns of signals to the electrodes which induce given signals in the pen in response thereto. The induced signals are applied to a detection circuit of the tablet. The relative position of the pen is determined by analysis of the signals presented and the signals detected. In a so-called coordinate detection period the control circuit of the known tablet presents a large number of successive patterns to the electrodes, each time one electrode then being given a voltage difference relative to the other electrodes. The signal induced in the pen is maximum at the instant at which the pattern with the voltage difference is presented to the electrode whereto the pen is nearest. Because of the presentation of the patterns, a series of code elements, represented by signal levels, is formed on each electrode during the detection period, said series being representative of the relevant electrode. The detection circuit can thus determine the relative position of the pen each time in a coordinate detection period. It is a drawback of the known tablet that the coordinate detection period is comparatively long, so that a comparatively long period of time is required to determine the instantaneous position of the signal sensor relative to the electrodes.